Many natural and biological/chemical industrial and municipality processes produce undesired constituents in a gas associated with wastewater. For example, hydrogen sulfide (H.sub.2 S) and other odorous constituents may be produced from certain types of wastewater. For protection of the environment and for public safety, such undesired constituents are desirably neutralized or reduced in concentration to a non-dangerous and, more preferably, to a non-offensive level, before discharge to the environment.
There are several common conventional approaches for reducing such undesired constituents in the gas associated with wastewater. For example, two approaches withdraw the gas to be treated from the region of generation, and transfer the gas to a scrubbing or cleaning apparatus. In one approach, an absorbent material is used to separate and collect the undesired gaseous constituents before release to the atmosphere. Unfortunately, this approach produces problems of disposal and/or renewal of the spent absorbent material.
A second approach reduces the undesired constituents or components of the gas by application of a solution of scrubbing chemicals and water to the gases flowing within a scrubber vessel. More particularly, scrubbing solution may be contained in a sump connected in fluid communication with a lower portion of the scrubber vessel. The scrubbing solution is withdrawn from the sump, sprayed through the interior of the vessel, and is returned to the sump for further recirculation. Diffusing bodies may also be included within the vessel to increase the area available for chemical reaction between the gas being treated and the scrubbing solution. The chemical processes involved are well known and result in reduced and/or neutralized undesired constituents of the gas discharged to the atmosphere. This second approach may be considered a typical preferred treatment approach.
Unfortunately, operation of a conventional scrubber apparatus may present problems of disposal of the spent scrubbing solution, and the cost of the required treatment chemicals. Moreover, a conventional scrubber may experience significant maintenance difficulties due to corrosion caused by the scrubbing solution and associated high humidity.
A conventional scrubber may also typically use a pH and/or conductivity sensor to measure the concentration and/or activity of the treatment chemical in the scrubbing solution. Treatment chemicals may be added to produce a selected pH and/or conductivity level in the scrubber solution. Accordingly, the control is not directly related to the concentration of the undesired gaseous constituents which may vary by a factor of 100 or more during a typical week (from less than 1 PPB to greater than 300 PPM).
Since pH and/or conductivity control cannot by itself adjust for the varying concentration of the undesired constituents in the gas, excessive treatment chemical consumption typically occurs. In other words, conventional control of the amount of treatment chemical delivered to the scrubbing solution is done rather grossly, and the tendency is to overdose the scrubbing solution with treatment chemicals. This excessive chemical consumption may cause fouling of the diffusing bodies, accelerate corrosion of the vessel and associated components, and require considerable make-up water to be added to the scrubbing solution. In addition, the gross control of the chemistry of the scrubbing solution may also result in failure to sufficiently reduce the undesired constituents with the results being release of noxious or toxic gas. Thus, such a conventional approach typically results in increased operational and maintenance costs, as well as increases the likelihood of damage to the environment.
A wet scrubber is also commonly used for other types of pollution control. For example, a wet scrubber for the metal casting industry is disclosed in U.S. Pat. No. 4,172,880 to Tzavos for removing gaseous pollutants by contact with an acid scrubbing solution. The scrubber is controlled responsive to the electrical conductivity of the scrubbing solution. Along these lines, U.S. Pat. No. 4,229,411 to Kisters et al. discloses an apparatus including sensors for measuring the concentration of various pollutants, and a computer for calculating the necessary amount of neutralizing agent which is introduced into the flow of gas being treated.
Another somewhat related approach to reducing undesired constituents in a gas associated with wastewater includes direct injection of treatment chemicals into the wastewater stream. Direct injection of treatment chemicals is disclosed, for example, in U.S. Pat. No. 5,356,458 to Javadi et al. The patent discloses a system for continuously monitoring the H.sub.2 S concentration above a wastewater stream, such as a sewage stream, and for controlling the amount and time of injecting the treatment chemical directly into the wastewater stream to control the H.sub.2 S which would otherwise escape or evaporate into the atmosphere. In particular, a gas sample pump moves a sample of gas through a sample line and to the H.sub.2 S sensor. Alternately, a sample line may be connected to draw water directly from the wastewater, with the entrained gas being released or removed before reaching the sensor. The H.sub.2 S sensor may be connected to a reference or calibration gas, and may be periodically purged by operation of various valves under control of a processor. Unfortunately, directly injecting a chemical into the wastewater stream may be impractical in many situations or may not effectively control the generation or release of H.sub.2 S from the wastewater.
Another difficulty with control of undesired constituents in the gas associated with wastewater is that conventional gas sensors, such as for H.sub.2 S, may be readily damaged or contaminated in the presence of high humidity and corrosive gases--particularly where it is desired to place the sensor in a duct carrying the gas. A conventional H.sub.2 S sensor, even if it were to initially survive such a harsh environment, may readily fail or lose calibration during its life. See, for example, G. D. Waltrip, E. G. Snyder, "Elimination of Odor at Six Major Wastewater Treatment Plants," Journal WPCF, Volume 57, Number 10, pg. 1031, October 1985.